Embossed battery separator



March 29, 1949. s. l. sTRlcKHousER Er/AL 2,465,493

Emaossn BATTERY snanon 2 sneet-sheet 1 Filed Dec. 5, 1944 LW L.nv n un uw uw un y 0n L? ..1 Lf? L0 2d d. n Ln .LT Lw LFIlIIH 0T un u uw O y 11mm/EY March 29, 1949. s. sTRlcKHousER E'rAL 2,465,493

` EMBOSSED BATTERY SEPARATOR Filed nee. 5, 1944 2 sheets-sheet 2 n WM Wauu?,

4 TTORVPY Patented Mar. 29, 1949 2,465,493 EMBOSSED BATTERY SEPARATOR Sherman I. Strickhouser, Edgewood, and Edwin C. Uhlig, Greenwood, R. I., assignors to United States Rubber Company, New York, N. Y., Va corporation of New Jersey Application December 5, 1944, Serial No. 566,712

7 Claims.

This invention relates to battery separators of microporous vulcanized hard rubber or other microporous plastic composition.

This application is a continuation-in-part of our prior application Serial No. 501,984, filed September 11, 1943, which has been abandoned.

Thin porous separators made of non-conducting material with suicient mechanical strength for convenient handling are placed between alternating positive and negative plates in storage battery cells to prevent metallic conduction between the plates of opposite polarity while freely permitting electrolytic conduction.

These separators preferably are made as thin as is consistent with adequate strength and insulating properties and are provided on the face exposed to the positive plate with raised ribs that stiften the separator and provide greater opportunity for circulation of the electrolyte at the face of the positive plate.

separators, when soaked in storage battery electrolyte, should have as low resistance to electrolytic conduction as it is possible to secure and at the same time afford adequate protection against metallic conduction between the plates, since the lower the resistance to electrolytic conduction the better the battery will perform.

A type of battery separator extensively used in recent years is formed of a microporous hard rubber plate having a at, smooth surface upon one face and raised ribs upon its other face. This separator has been manufacturedl by producing an unvulcanized sheet having ribs upon one face thereof and then vulcanizing the ribbed sheet, but it is diicult to vulcanize the sheet after the ribs are formed without flattening or otherwise distorting the ribs, unless a temporary or permanent support is provided to protect the ribs during vulcanization.

While separators constructed in the manner just described and having thin Web areasv and thick rib areas are extensively used, the ribs increase the resistance to electrolytic conduction, and the operation above referred to for protecting the ribs during vulcanization adds considerably to the cost of manufacturing the separators. Furthermore, such separators'when finished frequently have to be ground to bring them accurately to the required overall thickness.

The present invention contemplates an extremely simple and practicable construction for microporous battery separators that have pro- (Cl. 13G-145) is to provide separators of microporous vulcanized hard rubber or heat-softenable microporous plastic composition, or other fairly rigid microporous plastic material suitable for battery'separator construction and having stiiening ribs upon one face, but which separators are so constructed that the rib areas have as low resistance to electrolytic conduction as the flat web areas of the separators. Such a construction is secured by forming the separator with raised ribs upon one` face and corresponding depressions upon the opposite face. This construction permits good circulation of the battery electrolyte at the faces of both the positive and negative plates.

Another important object of the present invention is to improve the construction of such battery separators whereby they may be made more uniform in overall thickness than heretofore and more economically than the microporous ribbed separators provided heretofore. A

Another object of the present invention is to provide battery separators that will be slightly resilient under lateral compression so that they will t snugly between the battery plates and prevent the separators and plates from vibrating Within a battery during its normal use. Another object is to so construct the separator ribs that the ends of the separators may be easily stuffed in place between the negative and positive plates of a battery cell. Another object is to form a separator so that the ribs are omitted from adjacent tothe ends of the separator so as to lessen the tendency of the ends to crack; and to round the corners o f the separator so as to reduce their tendency to chip or crack.

When hard rubber or other plastic materials such as herein contemplated are heated, while being subjected to a constant applied load, a temperature is reached at which such material tends to lose its rigidity and permits slight bending under such load, the temperature at which this occurs is called the softening temperature or the softening point and is easily determined by using an A S T M test. As the temperature increases further the material becomes much softer and its ability to resist deformation under stress decreases rapidly. This temperature above the isoftening point at which the material is sui- 3 ciently softened to yield quickly without rupturing is usually referred to as the yield temperature and will be so referred to hereinafter.

In order to produce the battery separators of the present invention a heat-softenable microporous sheet of plastic composition is heated to a point'between its yield temperature and fusing temperature, and while so heated the sheet is subjected to an embossing operation that pro- Y 1o The portion of the separator lying between the duces raised ribs upon one face of the sheet and corresponding identations at the other face of the sheet.

Battery separators having the construction contemplated by the present invention can be producednot only by the sheet embossing operation just mentioned, but alsoby a molding operation carried out by introducing into a suitable mold a heat-hardenable or heat sotenable material in the form of a batt of plastic laments, or a shapeless mass of such material containing a pore-forming agent, or small plastic particles of such material in suiiicient volume to allow formation of a sintered product. The nished separator contemplated by the present invention should have a porosity greater than 40% and a yield temperature above 100 F.

The above and other objects of the presentv bossing mold shown slightly open and with the sheet that is to be embossed in the mold;

Fig. 6 is a similar view showing the mold closed with the sheet embossed therein; and- Fig. 7 is a view similar to Fig. 5 showing the mold open and the embossed sheet lying in spaced relation between the mold plates.

The battery separator herein contemplated is .Y

provided with ribs adapted to supply added strength and to space the separator from the positive plate, and these ribs are so formed that the thickness of the'sheet material of the separator in the rib areas is not greater than that of the web areas, with the result that the electrolytic conduction is very nearlyuniform throughout all parts of the separator. The ribs may be p given various sizes and shapes and in the present construction vertical rows of short ribs are provided throughout the major portion of the separator, since short aligned ribs separated by small flat web areas give better resistance to deformation than long continuous ribs. Also the ends of some of these short ribs are disposed in staggered relation to each other in the body 'portion of the separator, and all ribs terminate short of the upper and lower ends of the separator. The ribs at the marginal side edges of the separator are preferably made continuous to lessen the tendency of the electrolyte to ilow laterally beyond the battery plates. 'i

Now referring to the drawings and more particularly to Fig. 1, there is shown abattery separator I formed of microporous vulcanized hard rubber -or other microporous'plastic material and which is provided near its side `marginal edgesA .45 tion to receive the separators.

with relatively wide ribs formed of the two closely spaced ribs I the ends of which are connected to the off-set looped portions I2. In this manner a continuous rib or double rib Is provided at each side of the separator which serves to stiien a relatively wide marginal portion of the separator where each side extends beyond the side edges of the adjacent positive and negative plates.

the aligned ribs I3. These portions i6 help to prevent the entire separator sheet from spreading laterally when pressure is exerted against the ribs. v

A highly desirabie feature of the present separator resides in the construction whereby all ribs terminate short of the upper and lower ends of the separator as shown in Fig. 1 sov that a fiat margin is provided at each end of the separator that is less likely to break, and the corners I5 .of the separators are preferably rounded as shown so that these corners-will be less likely to chip Another highly desirable feature of the present separator `resides in the construction whereby the ribs near each end of the separator are made slightly lower than the rest ofthe ribs i3 as indicated by I3 in Fig. 4 so that the over all thickness of the separators will be reduced slightly near each end. The purpose of this is to facilitate the introdction of these separators in place between the positive and negative plates of a battery in making up an assembly of plates and separators to be placed in a battery cell. The positive platesprovided in a battery cell are all secured to a cross strap andthe negative plates for such cell are all secured to a second cross strap.- After these positive and negative plates have been assembled in alternate relation they may be sprung apart somewhat in tanned rela- By making the ribs I3 lower ortapered the separators can be easily stued in between the plates, whereupon the platesand separators can be'pressed together for insertion into the usual battery cell.

The battery separator shown in Fig. 1 is preferably formed by providing a ilat, smooth' sheet of microporous vulcanized hard rubber or4 other heat-softenable microporous plastic material having a yield temperature above 100 F. and indicated by I6 in Fig. 5 of the drawing. The sheet IB is preferably heated to slightly above its yield temperature, which will vary for different stocks.

This is preferably done by placing the sheet IB l in hot water say at a temperature of 200 F. for

a sheet having a yield temperature of approximately 160 F. and after remaining in this bath say for some minutes, it is placed while hot and wet in the position shown in Fig. 5 between the male and female molding plates I'l and I8 of the mold, which molding plates are preferably heated say to a temperature of 170 F. for a sheet having the yield temperature just mentioned. The molding plates may be conveniently maintained at this temperature by providing each of the plates I1 and I8 with an electrical heating coil I9. 'The lower plate IT is provided with the embossing ribs 20 and the upper. plate I8 is provided with the correspondingly shaped depressions 2|. In

the construction shown the ribs 20 and depressions 2l have-sharp angles in cross-section, but

the sheet I6 when embossed by these ribs will be providedA with the rounded ribs I3 due to the clearance space between the ribs 20 and depres sions 2I. The ribs 20 and depressions 2l may, if desired, be given the rounded contour of the rib I3.

The mold of Fig. 5 having the construction just described and having the sheet I6 therein is closed under pressure for a short period, say thirty seconds, to emboss the softened sheet I6 with the ribs as shown in Fig. 6, but the mold should not be fully closed until the sheet I6 has beenheated to or above its yield temperature. The extent to which the mold may be closed is limited by the marginal edge portions 22 of the mold which serve as stops'.` As soon as the desired ribbed construction is imparted to the plate I8, the mold is opened as shown in Fig. 7 so that the ribbed separator may be removed therefrom.

It is important that as soon as the mold is openedand the plate I6 is removed that this embossed plate be cooled quickly below the yield temperature, for if the plate is not cooled quickly after the mold is opened, the embossed design may be partly lost by the return of the soft sheet I6 toward its original at condition and the accuracy of the embossing operation thus reduced. This tendency of the hot plastic sheet to return towards its original flat condition can also be prevented by cooling the mold with the sheet I6 therein.

It is found when using a material7 such as microporous vulcanized hard rubber ving a yield temperature below the boiling point of water, that wetting the plate I6 with water before it is introduced into the mold facilitates the molding operation by causing a rapid heat transfer from the mold to the sheet, and also helps to cool the plate quickly after it is removed `from the mold. If, however, the plastic material used has ahigher yield temperature than 212 F. then the wetting uid used should have a higher boiling point than water, in this case glycerol or butyl alcohol (l-butanol) may be used.

It is important'that the molding ribs 20 and corresponding depressions 2| be so shaped that vwhen the mold is closed the softened sheet I6 will be distended in the rib areas so that the thickness of the sheet material in this area after such molding operation will not exceed the thickness of the at web portion of the sheet I6 as will be apparent from Fig. 3 of the drawings. In fact the' stretching or distending of the material to form the rib I3 reduces the thickness of this sheet material slightly in the rib as shown in Fig. 3. As a result of this construction the electrolytic conductivity of the separator will be very nearly uniform throughout all parts of the separator.

While the separator i0 has been above described as having the ribs formed therein by an embossing operation performed by the cooperating flat plates I'I and IB of the mold, such ribs may be formed by passing the sheet I6between rotating heated rolls formed with cooperating ribs and depressions, or by an extruding operation.

This separator can also be produced by a molding operation carried out in the mold I1, I8 or other type of mold, by introducing therein a shapeless mass capable of hardening to form a microporous separator, or a. powder capable of being sntered to form a microporous separator. The plastic separator thus produced should have a yield temperature above 100 F.

It is possible by providing a microporous sheet I6 of rubber or plastic and then embossing the same in an accurately formed mold in accordance with the present invention, to form the ribs of the sheet with a high degree of accuracy so that the overall thickness of the separator will conform accurately to the desired dimensions,

thus making it unnecessary to subject the sepa-" Y make it microporous. In which case the finished separator will contain a hydrousI silica gel or the like as a. pore-forming material, and preferably will have a porosity of over 40% as measured by a water displacement method.

A separator constructed as shown in Fig. 1 permits the electrolyte at the face of the positive plate to pass laterally from rib to rib through the passages I4, and the channels formed by the indentations corresponding to the ribs I3 facilitate usually tight in a battery cell.

the now of the electrolyte at the face of the negative plate, and increase the space for the electrolyte at the face of the negative plate. Furthermore, while the hollow ribs I3 are capableV of resisting considerable crushing pressure, they are more yieldable than solid ribs and will yield slightly underlthe pressure of the positive and negative plates when. the battery parts fit un- This facilitates the assembly of these parts in the battery in snug-fitting relation to each other and prevents the separators and plates from vibrating in the battery.

'Ihe thickness of the microporous sheet I6 used to form the separator I0 may vary within a substantial range, a gage of .027" is considered good, and if this sheet is made as disclosed in the above cited Baty et al. patent or of a heatsoftenable plastic it should when formed into the present separator be suilciently non-brittle to handle well without cracking and sti enough to be readily stuffed in between the battery plates.

The microporous sheet of vulcanized hard rubber or plastic material should have an impact strength of at least 0.8 ft. lb. per inch of notch as determined bythe notched Izod test (ASTM D256-41T) and a brittleness consistent with the stiffness of the material so that the separator can be readily stuffed into a battery cell without breaking. The stiffness of the finished separator should be such that for a separator six inches square a compressive force of 7 ounces in a direction parallel to its length, or 4 ounces similarly applied to a side edge, at F., will not bend the separator enough to shorten it more than 1A of an inch.

The following is a list of heat softenable organic plastic materials that may be used to form the microporous battery separator of the present invention 1. Vulcanized hard rubberv (natural rubber) 2. Vulcanized hard rubber (synthetic rubber Buna S, Buna N) 3. Plasticized or non-plasticized-plastics (a) Polystyrenes .(Polybre, Lustron, Styron, #18 Styramic, Styramic HT, Cerex) (b) Polyacrylates (Lucite, polymethylmethacrylate) (c) Polyvinylidenes (Geon, Saran) (d) Polyvlnyls (Polyvinyl Chloride, Vinylites) (e) Polyamides (nylon) The plasticized or non-plasticized-'plastics are preferably l'lberized and collected in a batt or mat and then sintered at such pressure and temperature as to form a microporous sheet having a porosity greater than 40% and the other physical properties above described as necessary for the battery separator herein contemplated.l The porosity of such a sheet can be controlled by the diameter or angular disposition of the laments in the sheet and pressure and heat used to unite them. A microporous sheet of these plastics may also be formed by sintering small particles thereof under proper conditions.

Since, when rubber is used, the present invention contemplates vulcanizing the sheet I6 before the ribs are formed thereupon, it is a simple matter to wind long lengths of the sheet I6 or wider widths of the same onto a drum with a liner cloth and vulcanize a.4 large. quantity ofthe sheet, whereupon it may be cut into the strips I6 ready for the embossing or rib-forming operation which may be formed upon a long strip. The embossed strip may then be cut into the desired lengths for battery separators. This completes the operation of forming such separator except possibly for trimming the side edges adjacent the ribs Il to bring the separator accurately to the desired dimensions.

It will be apparent from the foregoing that we have developed a simple and inexpensive microporous battery separator having ribs which are so formed that the electrolytic conductivity of the separator will be lvery nearly uniform throughout its entire area, and which separator is suiciently non-brittle to handle Well without breaking while it has suiicient stiffness to be readily stuffed into a battery cell.

Having thus described our invention, what we claim and desire to protect by Letters Patent is:

1. A battery separator that can `be freely handled without breaking and capable of giving good service in a battery, comprising a microporous sheet of heat-softenable organic plastic composition provided with rows of raised distended ribs upon one face with corresponding indentations at the other face whereby hollow ribs are formed and all portions of the separator have approximately the same electrolytic conductivity, and said rows terminating short of the ends of the separator to lessen the tendency of such ends to crack.

2. A battery separator that can be freely handled without breaking and is stiff enough to be readily stuffedinto place in abattery cell and 1s capable of giving good service in a battery, comprising an embossed single-ply microporous sheet of heat-softenable organic plastic composition provided with raised distended ribs upon one face with corresponding indentations at the Vother face whereby hollow ribs are formed and all portions of the separator have approximately the same electrolytic conductivity, and said ribs being lower in height for a substantial distance near an end of the separator and tapered from one height which is greatest at the central portion of the separator to the other and also formed so that they terminate before they reach said end to make the separator easy to stui into place in a battery cell.

s handled without breaking and capable of giving good service in a battery, comprising a` microporous sheet of vulcanized hard rubber provided with rows of raised distended ribs upon one face with corresponding indentations at the other face whereby hollow ribs are formed and all portions of the separator have approximately the same electrolytic conductivity, and said rows terminating short of the ends of the separator to lessen the tendency of such ends to crack.

4. A battery separator that can be freely handled without breaking and is capable of giving good service in a. battery, comprising an emformed of a microporous composition selected from the group consisting of vulcanized hard rubbers, polystyrenes, polyacrylates, poly vinylidenes, polyvinylsf and poly amides, and provided with rows of rraised distended ribs upon one face with corresponding indentations at theV other face whereby hollow ribs are formed that terminate 'short of the ends of the separator and all portions of the separator have approximately the same electrolytic conductivity.

5. A battery separator comprisingv a microporous sheet that is constructed throughout of stiff heat-softenable organic plastic composition having a yield temperature greater than 100 E. and having raised distended ribs upon one face and corresponding -indentations at the other face I so that the thickness of the material of these ribs does not exceed the thickness of the non-ribbed portion of the sheet and the electrolytic conductivity is nearly uniform throughout all parts of the sheet having this construction, the major portion of the sheet being provided with rows of non-continuous aligned ribsthat terminate short of the ends of the separator.

6. A battery separator comprising a microporous sheet that is constructed throughout of stiif heat-softenable organic plastic composition having a. yield temperature greater than 100 F. and having raised distended ribs upon one .face and corresponding indentations at the other face so that the thickness of the material of these ribs does not exceed the thickness of the nonribbed portion of the sheet and the electrolytic conductivity is very nearly uniform throughout all parts of the sheet, the marginal side edge portions of the sheet being provided with ribs having laterally extending spaced U-shaped .portions that stiiens the sheet transversely and the rest of the separator being provided with rows of non-continuous aligned ribs that terminate short of each end of the separator.

7. A battery separator that can be freely handled without breaking and is stii enough to be readily stuied into place in a battery cell and is capable of giving good service in a battery,

comprising an embossed single-ply microporous 3. A battery separator that can be freely sheet that is constructed throughout of vulcanized hard rubber having a porosity of over 40%, and provided with rows of raised distended ribs upon one face only with'corresponding indentations at the other face, and said ribs being lower in height for a substantial distance near an end of the separator and tapered from one height which is greatest at the central portion of the separator to the other to make this end of the separator easier to stui into place in a battery cell.

SHERMAN I. STRICKl-IOUSER.

C. UHLIG.

(References on following page) REFERENCES CITED The following references are of record in the file of this patent: Y

UNITED STATES PATENTS Number Number Number Name Date Rudolph Sept. 29, 1942 Behrman Nov. 24, 1942 Baty et a1. Sept. 14, 1943 Hall Nov. 30, 1943 Strickhouser et al. Aug. 14, v1945 FOREIGN PATENTS Country Date Great Britain Aug. 17, 1937 OTHER REFERENCES Rupp, J. L., Electrical Engineering, Transactions section, November, 1944, page 774. 

